The present invention relates to a thermal interface and, in particular, to a compressible thermally-conductive interface.
Management of the operating temperature of modern electronic components is often a substantial challenge in that as more and more functionality is provided in electronic components of seemingly ever-decreasing size, the removal of heat generated by such component becomes more difficult. In addition, these electronic components are being made to operate at faster operating rates which also increases the heat generated by such components. Thus, the problem of heat removal is compounded by the combined effects of smaller size and greater operating frequency.
Conventional approaches to removing heat from electronic components involve the transfer of the heat to a thermal dissipating element, such as a heat sink, cold plate or other suitable heat removal means. In the prior art arrangement of FIG. 1, for example, a semiconductor die 10, which produces heat when operating, is coupled to a heat sink or heat spreader 20, which dissipates heat aided by heat sink fins 22. Die 10 is mounted to a circuit board 12 and is connected thereby to external apparatus (not shown) by solder ball or pin connections 14. Typically, die 10 is protected by an encapsulant 16, such as a molded plastic. To provide a heat transfer path between die 10 and heat sink 20, an interface of a thermally-conductive material is placed between die 10 and heat sink 20, and the collection of die 10, circuit board 12, heat sink 20 and interface 24 are mechanically fastened and held together, such as by clamps or clips 30. Typical conventional interface pads 24 may be conformable to an irregularly-shaped component, but they are not compressible, which limits their utility.
Heat transfer is often aided if the interface pad 24 undergoes a phase change, i.e. melt flows, at suitably low temperature under the pressure of clamps 30, thereby to better conform and intimately contact the surfaces of the heat generating component 10 and of the heat dissipating elements 20. Such interface pads 24 have little or no bonding strength and so must be secured mechanically, as by clamps, clips and the like. Conductive interface pads such as type CP7508 available from AI Technology, Inc. of Princeton, N.J., will flex and melt-reflow at about 50xc2x0 C. under normal clamping pressures, although they are not compressible and have little or no bonding strength to bond die 10 to heat sink 20. While the melting helps to eliminate trapped air spaces and voids between the two interfacing surfaces, even if some bonding strength develops after melt-reflow, that bonding strength drops essentially to zero each time the melt-reflow temperature is reached.
The lack of compressibility, low-temperature phase change and/or substantial bonding strength of conventional interface pads tends to limit their effectiveness and/or convenience of use in improving heat transfer across an interface. The need for mechanical clamps and fasteners can add as much as US$0.50-US$1.00 to the installed cost of a typical integrated circuit, such as a CPU for a computer.
While the conventional interface pads may work satisfactorily for a single component, such as a CPU chip, they are less effective for modules including plural components that can have a planarity tolerance of the heights of the various components as large as about 0.5 mm (about 20 mils) as illustrated in FIG. 2. A plurality of heat-generating and non-heat generating components 10 mounted to a circuit board or substrate 12 have top surfaces that are either at different heights or are tilted. Such differences in height arise, for example, from differences in the height of the components 10 and differences in their mounting to and placement on substrate 12. A conformable interface pad is flexible and so will tend to bend or drape across the differing height components 10, but is inherently incapable of being in intimate thermal contact with the heat sink 20 and both the taller and shorter components 10. A similar problem exists for a single component 10 that has an irregular or non-planar surface.
A compressible interface pad, if available, could accommodate such planarity tolerances. Prior art compressible interface materials include thermal greases and gels which are messy to use and require mechanical clips or clamps, and; elastomers that do not under go a phase change or melt-flow within the safe case operating temperatures of typical electronic components, and which often also require clips or clamps.
Accordingly, there is a need for a thermal interface material that provides the desirable characteristics of compressibility with a low temperature phase change or compressibility with low-temperature in situ curing to a substantial bonding strength.
To this end, the thermally conductive interface of the present invention comprises a blend of a compressible binder and at least one thermally conductive filler, wherein the compressible binder includes one of (a) a mixture of a thermoplastic rubber and a thermoplastic pressure sensitive adhesive, (b) a mixture of an epoxy blend and a curing agent for curing at less than 75xc2x0 C. and a thermoplastic pressure sensitive adhesive and (c) a mixture of an epoxy blend and a curing agent for curing at less than 75xc2x0 C. and a thermoplastic rubber or elastomer.